Two-dimensional electron gas (2DEG) generated in the vicinity of an interface of a channel layer (carrier transit layer) with a barrier layer (carrier supply layer) is used as a carrier in a high electron mobility transistor (HEMT) which contains a gallium nitride (GaN)-based material. The HEMT containing the GaN-based material (GaN-HEMT) is promising for a high-frequency element and a high-voltage element. A cap layer is formed on the barrier layer for improving characteristics such that suppression of current collapse and suppression of gate leakage by relaxing an electric field concentration.
However, the cap layer inevitably contains a donor impurity such as oxygen (O), and leakage current caused by the donor impurity flows. It is possible to offset the donor impurity when iron (Fe), being an acceptor impurity, is contained in the cap layer so as to reduce the leakage current, but the characteristics of the HEMT whose cap layer contains Fe are difficult to be stabilized.
Patent Literature 1: Japanese Laid-Open Patent Publication No. 2012-28706
Patent Literature 2: Japanese Laid-Open Patent Publication No. 2014-72430